Award: OCE-1735846

This project investigated the controls on and effects of blooms of calcifying phytoplankton in the Subantarctic. Phytoplankton blooms in this region tend to be dominated by coccolithophores. Regarding the controls on the coccolithophore blooms, this project points to a key role for temperature in mediating growth rates. Field observations suggest that the bloom is triggered by seasonal warming. These results are important because they shed light onto the physiological controls of phytoplankton community composition. Phytoplankton community composition is a key control on the biological pump. Calcifying organisms are particularly important as they export alkalinity from the surface ocean, thereby reducing the surface ocean buffer capacity and its capacity to absorb CO2 from the atmosphere. We also conducted numerical experiments with a global ocean biogeochemistry model. The results of this study demonstrate that Subantarctic calcification has the potential to trap alkalinity in the Southern Ocean, thereby exerting influence on alkalinity concentrations at the surface ocean in lower latitudes. When Subantarctic calcification is increased, greater amounts of alkalinity are trapped in the Southern Ocean; this reduces alkalinity concentrations at low latitudes and reduces the uptake of CO2 from the atmosphere. Conversely, reducing Southern Ocean calcification lets more alkalinity leak out of the Southern Ocean, thereby increasing low latitude CO2 uptake. Understanding these dynamics is important to understand how the ocean functions as a biogeochemical system, sustain marine ecosystems and mediating the carbon cycle. Last Modified: 10/27/2023 Submitted by: Matthew Long